Circuit arrangement



April 11, 1939. c. TRAVIS CIRCUIT ARRANGEMENT Original Filed April 27, 1933 3 Sheets-Sheet l INVENTOR cHA Es TRAVIS BY A'I'I'ORNEY April 11, 1939. c. TRAVIS 2,153,778

CIRCUIT ARRANGEMENT Original Filed April 27, 1933 3 Sheets-Sheet 2 A. 1 c. +250u B/AS INVENTOR CHARLES TRAVI3 ATTORNEY April 11, 1939.

c. TRAVIS 2,153,778

G IRCUIT ARRANGEMENT Original Filed April 27, 1953 3 Sheets-Sheet 3 VVV I INVENTOR CHARLES TRAVIS A'I'I'ORNEY Patented Apr. 11, 1939 UNIT STATES PATENT OFFICE CIRCUIT ARRANGEMENT Charles Travis, Philadelphia, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application April 27, 1933, Serial No. 668,231 Renewed October 1, 1937 10 Claims.

An object of the present invention is to provide a circuit arrangement whereby a vacuum tube oscillator having a plurality of grids will generate oscillations of substantially constant frequency.

A further object is to'provide such a tube oscillator in which the oscillation strength orvoltage generated may be maintained substantially constant.

A still further object is to provide a combined oscillator detector, which may for example be the first detector of a superheterodyne receiver, in which there is substantial freedom from frequency drift.

Another object of the invention is to further neutralize the capacity existing between the plate and a grid of a vacuum tube having a plurality of grids, including a screen grid. Still other ob:- jects of the invention will be apparent to those skilled in the art as the description thereof proceeds.

Circuit arrangements showing embodiments of the invention are illustrated in the accompanying drawings in which:

Figs. 1 and 2 show a vacuum tube having a plurality of grids arranged as a combined oscillator and detector;

Figs. 3 and 4 show circuits embodying my invention in connection with a combined oscillator detector having five grids;

Fig. 5 shows a circuit similar to that of Fig. 1 with a neutralizing arrangement according to my invention; and,

Fig. 6 shows an audio input circuit for a vacuum tube having a plurality of grids.

This invention relates in general to circuit arrangements useful in connection with a vacuum tube having several grids as disclosed in the application filed in the namev of J. C. Smith on January 31, 1933, Serial No. 654,421 and is in certain respects, an improvement on the circuit arrangements disclosed therein.

Referring to Fig. 1 a vacuum tube T is shown having a heater H which heats the electron emitting cathode C, and a plate 6. Interposed or located in the electron stream between the cathode and plate are a seriesof grids l, 2, 3, 4 and 5 arranged in the order shown, grid 1 being nearest the cathode and grid 5 nearest the plate 6. A tunable input circuit consisting of secondary 22 and variable condenser I8 is connected to the signal control'grid 4, the lead 20 connected to the opposite side of coil 22 being connected to the output circuit of the second detector of the receiver as shown, for example, in application Serial No. 642,544, filed Nov. 14, 1932, granted March 16, 1937, Patent No. 2,074,014 in the names of K. Chittick and W. L. Carlson. Or lead 20 may be connected to any other source of automatic volume control bias, as well known in the art to control the gain of the tube. Radio fre- 5 quency energy of any desired frequency, such as 500 to 1500 kilocycles, may be applied to the input terminals 19 of primary 2!. As shown, the local oscillator circuit includes coil II and variable condenser IZ by which the oscillation fre- 1o quency may be adjusted to produce any desired beat frequency. This circuit may be connected in a known manner to the oscillator control grid I through a condenser 10, a suitable grid leak resistor 9 being provided and connected to grid l and the cathode C. The normal steady bias of the grid 4 may be provided in known manner by a resistor 1 connected between cathode C and ground, 1 being shunted by a condenser 8.

The plate 6 may be connected to a. constant voltage source of about 250 volts through a parallel circuit comprising coil 16 and condenser H, the latter of which may be adjustable to tune the plate circuit to the desired beat frequency, which may be, for example, 175 kilocycles. The oscillator plate-grid 2 is shown connected to the 250 volt source through a resistor 14 shunted by a condenser l5 and a tickler coil l3 which is inductively coupled to coil II. This coupling between coils 13 and II provides the necessary feedback of energy to the grid circuit l, to cause the tube to generate oscillations. The screen grids 3 and 5 may be connected together conductively either within or without the tube and may have a steady positive voltage of 50 to 100 volts applied thereto from any desired voltage source (not shown) by a lead 23.

A tube of the type described above, has recently been put on the market by RCA Radiotron Company and is known as Model RCA 2A7. Now, I have found that, when the feedback circuit shown in Fig. 1 is used,'as the signal control grid 4 is biased back more negatively by the A. V. C. circuit, the local oscillation strength increases, that is, the magnitude of the locally produced oscillations increases. I also found that the oscillation frequency changes. In one instance I found that with a typical oscillator coil tuned by condenser I2 to 1,000 kilocycles, the frequency decreased 500 cycles per second when the bias on grid 4 was changed from 3 volts to cut-01f (about -30 to -35 volts).

The circuit arrangement shown in Fig. 2 is essentially the same as that of Fig. 1 except that in this case the tickler coil I3 is also included in the circuit of plate 6 so that the plate current of the tube flows through it, in addition to the current of oscillator plate-grid 2. With this arrangement I found that the oscillation strength or magnitude decreases as the control grid 4 is given a more negative bias. I also found that at an initial oscillator frequency of 1000 kilocycles, the frequency increased by 700 cycles as the bias on grid 4 was changed from -3 volts to about 30 volts. 4 In order to secure substantially constantoscillation strength and frequency as a variable bias is applied to control grid 4, I take a portion only of the plate feedback and combine it with the feedback of the oscillator plate-grid 2. This may be accomplished as shown in Fig: 3 by tapping the feedback coil l3 at such a place that the two opposing effects above mentioned balance out. With an RCA 2A7 tube I found that the turns in the lower tapped portion l3" were one third of the total number of turns in the entire tickler coil I3.

By using two separate tickler coils, each inductively coupled to coil l l as shown in Fig. 4, I was able to secure a similar result. By making the tickler coil 28 in the plate circuit of one third the number of turns of tickler coil l3, it was found that the oscillation strength and frequency remained substantially constant, as the bias on signal grid 4 was changed over a substantial voltage range. In the particular tube used in these tests, the oscillator control grid I is of the sharp cut-off type whereby less voltage is required to produce the desired oscillation strength, whereas, grid 4 was constructed to give remote cut-off or produce a result similar to that of a variable mu tube.

In Figs. 3 and 4 I have shown the intermediate frequency tuned circuit I6, I! inductively coupled to a secondary 24 tuned to the intermediate frequency by a condenser 25. This circuit may be connected as the input circuit of an intermediate frequency amplifier 21, being connected to the grid 26 thereof. For broadcast reception circuits I6, I! and 24, 25 are preferably tuned to 1'75 kilocycles or other desired beat frequency. To secure this frequency condenser I2 is initially adjusted so that the oscillation frequency is higher or lower. than the incoming signal frequency by an amount equal to the desired beat frequency. As applied to a superheterodyne receiver therefore, I am enabled by this invention to prevent detuning of the receiver due to change or. drift in the oscillator frequency caused by change in the grid bias of the signal control grid 4. Also, it will be understood that it is equally useful where the bias of grid 4 is changed by a manual volume control means in lieu of the A. V. C. means shown.

It will also be understood that the invention is useful where a tube working on the principle of electronic coupling, such as RCA 2A7, is to be used to modulate the locally generated oscillations at an audio frequency. Fig. 6 shows the circuit of control grid 4 of Figs. 3 and 4 arranged to effect this result. The input terminals 19 may be connected to the primary 42 of audio frequency transformer 40 the secondary 4| of which has its high potential end connected to signal grid 4. With this input circuit, the arrangement of Figs. 3 and 4 will generate oscillations of substantially constant radio frequency which will be modulated by the audio frequency voltage applied to terminals l9 from any desired source of such frequency.

In such a modified arrangement, it will be understood that the plate circuit need not be tuned by condenser II.

A further aspect of the invention is disclosed in Fig. 5 where the circuit shown is generally similar to that of Fig. 1. A neutralizing circuit has been added as shown comprising condenser CN and a coil 29 inductively coupled to coil 22. Coil 29 may consist of a few reversed turns wound around the low potential end of the radio frequency secondary coil 22. Even in the presence of the screen grid 3, 5 in a tube of the type above described, it is found that a capacity of about one quarter micro-microfarad (0.25) exists between plate 6 and grid 4, and it is the purpose of the neutralizing circuit shown to neutralize this capacity, At the radio frequency signal voltage applied to grid 4, the load in the plate circuit is essentially capacitive. The effective plate circuit capacity is always less than the physical capacity I 7 used to tune coil IE to the intermediate beat frequency. Calling f ==intermediate frequency, f2=the applied radio signal frequency; C=value of condenser l1 and C =eifective capacity in the plate circuit For alow applied radio signal frequency of say 550 kilocycles and using a high intermediate or beat frequency, of say 465 kilocycles, the value of Cp may be much smaller than C. Now it is very nearly exact to write:

equivalent to 250,000 ohms. The effect of this is substantially equivalent to connecting a resistor of this value across the coil 22 of the tuned signal circuit, and it may seriously affect both the gain and selectivity of the receiver. Or the effective plate circuit capacity C may be much smaller, as would be the case if the intermediate frequency were higher than 465 kilocycles, thus approaching more nearly the lower frequency end of the broadcast band, say 525 kilocycles, Further, as the capacity to be neutralized is very. small, and its effect is degenerative and not regenerative, a neutralizing condenser of fixed value may be used; that is, the circuit is not one requiring individual adjustment of the neutralizing condenser for each individual receiver. I

While the neutralizing circuit described has been shown as applied to a circuit as shown in Fig. 1 it will be understood that the same circuit may be employed in connection with the circuits shown in Figs. 3 and 4, the coil 29 being inductively coupled to coil 22 and the neutralizing condenser CN being connected to plate circuit 6 in the manner described,

In the circuits shown in Figs. 3 and 4 I found the following values to give satisfactory results when the source of voltage for plate 6 was 250 volts, voltage applied to screen grid 3,5:50 to 75 volts, voltage applied to plate-grid 2:100 volts. This latter voltage may be easily secured as shown by using a resistor [4 having a value of approximately 20,000 ohms. 1 I I While I have described my inventionin connection with a specific tube having several grids Grid conductance G,,, X

=4 micromhos it will be understood that its application is not limited to this tube, but it is applicable to any vacuum tube arrangement having at least two control grids and in which it is desired to maintain the oscillation frequency or amplitude, or both, substantially constant.

Having thus described my invention, what I claim and desire to secure by Letters Patent is:

1. In combination, in a radio frequency oscillation generator, a vacuum tube having a first grid and cathode, an input circuit tuned to a predetermined radio frequency connected to said cathode and first grid, a plate-grid and a plate in said tube, a circuit connected to said plate-grid including a coil conductively connected to said plate-grid and inductively coupled to said input circuit for feeding back energy to said input circuit to cause said tube to generate radio frequency oscillations, an output circuit tuned to a radio frequency connected to said plate and cathode, and an additional feed-back connection between said plate and input circuit.

2. In combination in a device of the class described, a vacuum tube having a cathode and grid, an input circuit connecting said cathode and grid and tunable to a predetermined frequency, a control grid, a plate-grid and a plate, means for applying a variable bias voltage to said control grid to control the currents passing to said plate, a circuit connected to said plategrid including means for inductively feeding back energy to said input circuit to cause said tube to generate oscillations, a plate circuit connected to said plate including additional means for feeding back energy to said input circuit, said first and second named feedback means being so proportioned that the frequency of the oscillations generated remains substantially constant as the bias voltage on said control grid is varied by a substantial amount.

3. In combination in a device of the class described, a vacuum tube having a grid and a cathode, an input circuit connecting said grid and cathode and tunable to a desired radio frequency, a control grid, a plate-grid and a plate, means for applying a variable voltage to said control grid to thereby control the current to said plate, a tapped coil inductively coupled to said input circuit to cause said tube to generate radio frequency oscillations, a connection from one end of said coil to a source of high voltage potential, a connection from the opposite end of said coil to said plate-grid, and. a connection from the plate of said tube to an intermediate point of said tapped coil.

4. The combination of the preceding claim in which the connection between the plate-grid and said coil includes a high resistance.

5. In combination in a device of the class described, a vacuum tube having a cathode and grid, an input circuit connecting said cathode and grid and tunable to a desired radio frequency, a control grid, a plate-grid and a plate, means adapted to apply a variable voltage to said control grid to thereby control the current passing to said plate, a pair of coils inductively coupled to said input circuit to cause said tube to generate radio frequency oscillations, a connection from one end of each of said coils to a source of high voltage potential, and separate connections from the other ends of said coils to said plate and plategrid.

6. The combination of the preceding claim in which the connection between the plate-grid and its connected coil includes a high resistance.

'7. An oscillation generator comprising the combination of a vacuum tube having a cathode, a first grid, a control grid, a plate-grid and a plate, an input circuit connected to said cathode and first grid and tunable to a desired oscillation frequency, means for applying a variable voltage to said control grid, a coil inductively coupled to said input circuit to cause said tube to generate oscillations, a direct connection between one end of said coil and a source of positive potential, a connection conductive to direct current between the other end of said coil and said plate-grid, and a circuit conductive to direct current connecting an intermediate point of said coil to said plate, the number of turns of said coil between said intermediate point and said first named end being substantially one third the total number of its turns.

8. ,The combination defined in the preceding claim in which a screen grid surrounds said control grid and serves to screen said plate-grid therefrom.

9. In an oscillation network of the type including a tube of the type provided with a cathode, an output electrode and at least two cold electrodes arranged in succession in the electron stream flowing from the cathode to said output electrode, a resonant circuit tuned to a wave frequency and connected between the cathode and one of said two electrodes, means providing feedback between said two electrodes to produce waves of said frequency, additional means providing feedback between said output electrode and said one electrode thereby to produce said waves, an output circuit connected to said output electrode, and additional means for varying the said electron stream at a frequency different from said wave frequency.

10. In an oscillation network of the type including a tube of the type provided with a cathode, an output electrode and at least two cold electrodes arranged in succession in the electron stream flowing from the cathode to said output electrode, a resonant circuit tuned to a wave frequency and connected between the cathode and one of said two electrodes, means providing feedback between said two electrodes to produce waves of said frequency, additional means providing feedback between said output electrode and said one electrode thereby to produce said waves, an output circuit connected to said output electrode, said tube including an auxiliary electrode in said electron stream, and means impressing a modulation potential on said auxiliary electrode to produce in said output circuit said waves in modulated form.

CHARLES TRAVIS. 

